1. Field of the Invention (Technical Field)
The present invention relates to probes for time domain reflectometry testing of electrical components.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The complexity of electronic components, particularly printed circuit boards, is steadily increasing, as is the demand for electronic components. These components need to be tested for faults, and high frequency test techniques such as time domain reflectometry provides one technique for much of such testing. Because of the high volume of needed testing, automated robotic systems are preferred in order to attain a high throughput for any testing system.
Time domain reflectometry (TDR) is a technique for detecting impedance mismatches and other characteristics of microwave transmission lines. A very fast (order 50 ps) rising edge is injected, via electric probe onto the line, and line characteristics such as characteristic impedance and propagation delay are calculated from the step response recorded on a digital storage oscilloscope (DSO). With the advent of fast bus structures in computer memory applications such as RAMBUS, traces on printed circuit boards (PCBs) must now be treated as distributed circuits or transmission lines. As a result, computer memory and backplane manufacturers are now being asked by memory bus and integrated circuit designers to control the characteristic impedance of PCB traces.
Unfortunately, existing time domain reflectometry robotic testing systems suffer from noise introduced by certain sources of imprecision in the systems themselves. For example, U.S. Pat. No. 5,994,909, to Lucas et al., discloses a robotic testing system having two probes 122,123 mounted on a single robotic arm wherein probe 123 is adjustable so that the distance between the two probes is adjustable. Such a mechanical system is prone to imprecision over time, because of vibration, temperature fluctuations, and like sources of distortion. Similarly, European Patent Application EP 0 953 844 A2 (corresponding to U.S. Pat. No. 6,051,978, to Swart) discloses a two-probe system but having a single probe on each of two robotic arms. Again, maintaining a precise desired distance between the two probes is difficult, again resulting is unwanted distortion of the testing results. Because of such imprecisions in existing systems, the shortest traces that can be tested accurately are those that are 1.5 inches or longer.
The present invention greatly increases precision in domain reflectometry systems by employing twin probes (signal and ground) having a fixed distance (pitch) between them and having certain other desirable electrical characteristics. Because testing often requires a number of different pitches, a robotic arm employing the invention is preferably able to select any of a plurality of fixed-pitch probes stored in a probe tip changer assembly, as disclosed in U.S. patent application Ser. No. 09/738,044. Because this prevents inaccuracies induced by changes in pitch over time plaguing prior art mechanical systems, the present invention is able to successfully test traces as short as 0.5 inches. The shapes of the probe tips and the preferred rectangular shape of the probe tip assembly preserve accuracy by precisely matching the electrical characteristics of circular coaxial structures such as coaxial cables. The present invention has a number of other advantages over the prior art as discussed below.
U.S. Pat. No. 4,740,746, to Pollock et al., discloses a controlled impedance microcircuit probe with a resilient center electrode in which impedance control is provide by maintaining a constant thickness of a dielectric between the center and the outer probe. The present invention does not have such limitation because air is used as a dielectric medium. Pollock et al.""s probe is also disadvantageous in that the compliance of the center electrode, provided by a multi-turn coiled spring, adds inductance to the ground circuit, thereby slowing the response time of the probe.
The following references disclose other electronic device probes having varying advantages and disadvantages: U.S. Pat. No. 5,959,460, to Johnson; U.S. Pat. No. 5,565,788, to Burr et al.; U.S. Pat. No. 5,506,515, to Godshalk et al.; U.S. Pat. No. 5,477,159, to Hamling; U.S. Pat. No. 4,829,242, to Carey et al.; U.S. Pat. No. 4,743,839, to Rush; U.S. Pat. No. 4,686,463, to Logan; U.S. Pat. No. 4,593,243, to Lao et al.; U.S. Pat. No. 4,116,523, to Coberly et al.; and U.S. Pat. No. 4,055,805, to Ardezzone. The following references disclose robotic testing systems having varying advantages and disadvantages: U.S. Pat. No. 6,008,636, to Miller et al.; U.S. Pat. No. 5,781,021, to Ifani; U.S. Pat. No. 5,498,965, to Mellitz; U.S. Pat. No. 6,024,526, to Slocum et al.; U.S. Pat. No. 5,844,412, to Norton; U.S. Pat. No. 5,696,450, to Itoh; U.S. Pat. No. 5,498,964, to Kerschner et al.; U.S. Pat. No. 5,469,064, to Kerschner et al.; U.S. Pat. No. 5,105,147, to Karasikov et al.; U.S. Pat. No. 4,881,863, to Braginsky; U.S. Pat. No. 5,631,856, to Keller et al.; U.S. Pat. No. 5,394,348, to Abe; U.S. Pat. No. 5,043,910, to Chiba; U.S. Pat. No. 4,628,464, to McConnell; and U.S. Pat. No. 4,593,820, to Antonie et al.
The present invention is of a coaxial probe for testing of planar electric transmission line structures comprising: a probe mount comprising a coaxial connector; a center electrode mounted on the probe mount and electrically connected to a center conductor of the coaxial connector, wherein the center conductor may be placed in contact with a first point on a planar electric transmission line structure to be tested; an outer electrode mounted on the probe mount and electrically connected to ground, the outer electrode comprising a protrusion to be placed in contact with a second point on the planar electric transmission line structure to be tested; and a dielectric of non-uniform thickness between the center and the outer electrodes. In the preferred embodiment, the probe mount comprises a conductive plate and the dielectric comprises air. If the probe mount comprises a printed circuit board, the dielectric comprises the printed circuit board and air, and the printed circuit board may comprise one or more stubs for tuning electrical characteristics of the probe, and one or more shorting bars located along the one or more stubs. The outer electrode preferably comprises a conductive tube having a non-circular cross-section, such as oval, square, rectangular, hexagonal, L-shaped, or U-shaped. The protrusion may be placed at any point on a downward-facing surface of the outer electrode without substantially altering impedance characteristics of the probe. The pitch between the center electrode and the protrusion is preferably fixed. The protrusion preferably comprises a 60-degree point. The outer electrode may be axially spring-loaded to provide compliance. The connector may be spring-loaded to provide compliance, preferably via a short-throw conductive spring. The probe can be constructed to be handheld during testing of the planar electric transmission line structure. Impedance characteristics of the probe substantially match those of a coaxial cable attached to the connector. Lumped series resistance (such as a resistor) may be attached to the outer electrode to increase speed of the probe.
The invention is also of a differential coaxial probe assembly comprising two probes as described in the preceding paragraph.
The invention is additionally of a coaxial probe for testing of planar electric transmission line structures comprising: a probe mount; a center electrode mounted on the probe mount, wherein the center conductor may be placed in contact with a first point on a planar electric transmission line structure to be tested; and an outer electrode of non-circular cross-section mounted on the probe mount. In the preferred embodiment, the outer electrode comprises a protrusion to be placed in contact with a second point on the planar electric transmission line structure to be tested. The protrusion may be placed at any point on a downward-facing surface of the outer electrode without substantially altering impedance characteristics of the probe.
The invention is also of a differential coaxial probe assembly comprising two probes as described in the preceding paragraph, with fixed relative positions, with manually variable relative positions, or with automatically variable relative positions, and with either zero or one protrusion on a downward facing surface of each of the outer electrodes.
The invention is yet further of a coupled line differential probe assembly comprising: a probe mount; two center electrodes mounted on the probe mount, wherein both of the center conductors may simultaneously be placed in contact with first and second points on a planar electric transmission line structure to be tested; and an outer electrode of non-circular cross-section mounted on the probe mount, the outer electrode comprising zero, one, or two protrusions to be placed in contact with additional points on the planar electric transmission line structure to be tested. In the preferred embodiment, the protrusions may be placed at any point on a downward-facing surface of the outer electrode without substantially altering impedance characteristics of the probe. The outer electrode can comprise one protrusion to be placed in contact with a third point on the planar electric transmission line structure to be tested, or zero protrusions whereby a common ground is not provided between said outer electrode and the planar electric transmission line structure to be tested.
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.